Line trimmers are widely used in maintaining lawns and gardens. Line trimmers cut vegetation by rapidly rotating one or more lines about a generally vertical axis in a generally horizontal plane. The rapidly rotating line cuts vegetation upon contact. As the line trimmer is operated, occasional unavoidable contact with the ground, stones and other objects occurs which results in abrasion or even breakage of the line. Consequently, the line must be replenished periodically.
Various line trimmer heads have been developed to provide an automatic line feed feature. A common type of automatic line feed mechanism is referred to as a "bump feed" wherein the line trimmer head is bumped against the ground to feed line from the line trimmer head. When the line trimmer head is bumped against the ground, a spool is unlocked for rotation relative to the line trimmer head. The bumping action axially shifts the spool support mechanism and causes it to overcome a biasing spring which normally holds the spool in an interlocked relationship relative to the line trimmer head. The bump feed approach to feeding line presents problems in that when the trimmer is bumped against the ground, vegetation is cut too close to the ground. Further, bump feed devices must be built to withstand constant shock loads caused by the bump feeding technique which can result in wear and tear on the line trimmer device. It is also difficult to control the rate of line advance with bump feed devices.
Other approaches to automatic line feed have attempted to incorporate the use of centrifugal force to cause periodic incremental line feed. On example of such a system is disclosed in U.S. Pat. No. 4,667,410 to Weid which discloses an automatic feed line trimmer head having an axially shiftable element which is shifted by movement of a ball bearing in an inclined ramp. When the motor is turned on and off, line feeds out incrementally. A sleeve shifts in one axial direction with respect to the head when the head begins to rotate. Teeth on the sleeve engage corresponding teeth formed on the interior of the spool. The teeth of the Weid device are subjected to high shear forces that may cause the teeth to break.
Another example of a ball bearing shifting mechanism in a line trimmer head is disclosed in U.S. Pat. No. 4,566,189 to Muto. The Muto device discloses the concept of providing a hub including a plurality of balls disposed in separate tracks. As centrifugal force drives the balls radially outwardly in the tracks, the hub shifts axially against the force of a biasing spring. A line supply spool has a series of projections which engage corresponding projections on the head. One problem encountered by such ball bearing devices is that the line trimmer heads are constantly exposed to dirt and debris which may infiltrate the line trimmer head and cause problems in regard to the movement of the balls. For ball bearing devices to operate, the ball bearing tracks must be kept clean or rough operation will result. Manufacture of ball bearing shifted link trimmer heads is complicated by the need to assemble the ball bearings in tracks. Further, servicing the devices in the field can be complicated by the loss of the ball bearings.
These and other problems encountered by prior art line trimmer heads are addressed and solved by the automatic line trimmer head of the present invention.